Method of depositing a coating containing silicon and oxygen

ABSTRACT

The present invention relates to the deposition of coatings containing silicon and oxygen from vaporized hydrogen silsesquioxane resin. The process comprises introducing the hydrogen silsesquioxane vapor into a deposition chamber containing the substrate to be coated and then inducing reaction of the vapor to form the coating.

BACKGROUND

The present invention relates to the deposition of a coating containingsilicon and oxygen from vaporized hydrogen silsesquioxane resin. Theprocess comprises introducing the hydrogen silsesquioxane vapor into adeposition chamber containing the substrate to be coated and theninducing reaction of the vapor to form the coating.

Numerous silica vapor deposition methods are known in the art and manyare currently used in industry. Typically, such deposition methodsinvolve decomposing a silicon source in the presence of oxygen and thesubstrate to be coated. For example, chemical vapor deposition (CVD) isused to decompose various silanes (e.g., SiH₄, H₂ SiCl₂, etc.) in thepresence of a source of oxygen (e.g., air, oxygen, ozone, NO₂, etc.) forthe deposition of protective or dielectric coatings.

Hydrogen silsesquioxane resin is likewise known in the art. Forinstance, Collins et al, in U.S. Pat. No. 3.615.272 describe such resinsand their method of manufacture. Similarly, Haluska et al, in U.S. Pat.No. 4,756.977 teach the use of hydrogen silsesquioxane resin to formceramic coatings on substrates. These patents, however, only describethe use of the resin in solution.

U.S. Pat. No. 5,063,267 assigned to the same assignee hereof, describesfractionating hydrogen silsesquioxane resin into narrow molecular weightfractions. The application describes the low molecular weight fractionsthereof as volatile but does not describe a utility for such materials.

The present inventor has now unexpectedly discovered that gaseous, lowmolecular weight hydrogen silsesquioxane can be used in conventionalvapor deposition techniques to form silicon and oxygen containingcoatings.

SUMMARY OF THE INVENTION

The present invention relates to a method of depositing a coatingcontaining silicon and oxygen on a substrate. The method comprisesintroducing a gas comprising hydrogen silsesquioxane into a depositionchamber containing the substrate. Reaction of the gas is then induced toform the coating.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that volatile fractionsof hydrogen silsesquioxane resin can be used to form coatings on varioussubstrates. The coatings produced by the techniques described herein arevaluable as protective and dielectric layers on substrates such aselectronic devices.

The term hydrogen silsesquioxane resin or H-resin is used in thisapplication to describe various hydridosilane resins of the formulaHSi(OH)_(x) (OR)_(y) O_(z/2), in which each R is independently anorganic group or a substituted organic group which, when bonded tosilicon through the oxygen atom, forms a hydrolyzable substituent,x=0-2, y=0-2, z=1-3, x+y+y+z=3. Though not represented by thisstructure, these resins may contain a small number of silicon atomswhich have either 0 or 2 hydrogen atoms attached thereto due to variousfactors involved in their formation or handling.

The above hydridosilane resins are generally produced by a processcomprising the hydrolysis and condensation of silanes of the formulaHSiX₃, wherein X is a hydrolyzable group. These reactions may result ina fully condensed (HSiO_(3/2))_(n) resin or the hydrolysis and/orcondensation may be interrupted at an intermediate point such thatpartial hydrolyzates (containing Si--OR groups) and/or partialcondensates (containing SiOH groups) are formed.

Various methods for the production of H-resin have been developed. Forinstance. Collins et al, in U. S. Pat. No. 3.615.272, which isincorporated herein by reference, describe a process of forming nearlyfully condensed H-resin (which may contain up to 100-300 ppm silanol)comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydratehydrolysis medium and then washing the resultant resin with water oraqueous sulfuric acid. Similarly, Bank et al, in U.S. Pat. No.5.010,159, which is hereby incorporated by reference, teach methods offorming such resins comprising hydrolyzing hydridosilanes in anarylsulfonic acid hydrate hydrolysis medium to form a resin which isthen contacted with a neutralizing agent. A preferred embodiment of thislatter process uses an acid to silane ratio of about 6/1.

Other resins, such as those described by Frye et al, in U.S. Pat. No.4,999,397, hereby incorporated by reference, those produced byhydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholichydrolysis medium or any other equivalent hydridosilane, are alsofunctional herein.

The above soluble hydridosilane resins are then fractionated to obtainthe low molecular weight species which can be volatilized in thedeposition process of this invention. Any conventional technique forfractionating the polymer can be used herein. Particularly preferred,however, is the use of a variety of fluids at, near or above theircritical point. This process is described in U.S. Pat. No. 5,063,267,assigned to the same assignee hereof, and is hereby incorporated byreference. The process described therein comprises (1) contacting theH-resin with a fluid at, near or above its critical point for a timesufficient to dissolve a fraction of the polymer; (2) separating thefluid containing the fraction from the residual polymer; and (3)recovering the desired fraction.

Specifically, the application describes charging an extraction vesselwith a sample of H-resin and then passing an extraction fluid throughthe vessel. The extraction fluid and its solubility characteristics arecontrolled so that only the desired molecular weight fractions ofH-resin are dissolved. The solution with the desired fractions ofH-resin is then removed from the vessel leaving those H-resin fractionsnot soluble in the fluid as well as any other insoluble materials suchas gels or contaminants.

The desired H-resin fraction is then recovered from the solution byaltering the solubility characteristics of the solvent and, thereby,precipitating out the desired fraction. These precipitates are thenmerely collected in a separation chamber by a process such as simplefiltration.

The extraction fluid used in this process includes any compound which,when at, near or above its critical point, will dissolve the fraction ofH-resin desired and not dissolve the remaining fractions. Additionalconsideration, however, is usually given to the critical temperature andpressure of the solvent compound so that unreasonable measures are notnecessary to reach the appropriate point. Examples of specific compoundswhich are functional include, but are not limited to, carbon dioxide andmost low molecular weight hydrocarbons such as ethane or propane.

By such methods, one can recover the desired fraction of H-resin. Otherequivalent methods, however, which result in obtaining the fractionsdescribed herein are also contemplated. For instance, methods such assolution fractionation or sublimation may also function herein.

The preferred fraction of H-resin to be used in the process of thisinvention is one which can be volatilized under moderate temperatureand/or vacuum conditions. Generally, such fractions are those in whichat least about 75% of the species have a molecular weight less thanabout 2000. Preferred herein, however, are those fractions in which atleast about 75% of the species have a molecular weight less than about1200. with those fractions in which at least about 75% of the specieshave a molecular weight between about 400 and 1000 (T8-T16) beingparticularly preferred. Additionally, it is contemplated that broadmolecular weight materials may be used herein as the source of H-resinvapor. Volatilization of such material, however, often leaves a residuecomprising nonvolatile species.

Once the desired fraction of H-resin is obtained, it is vaporized andintroduced into a deposition chamber containing the substrate to becoated. Vaporization may be accomplished by heating the H-resin sampleabove its vaporization point, by the use of vacuum, or by a combinationof the above. Generally, vaporization may be accomplished attemperatures in the range of 50°-300° C. under atmospheric pressure orat lower temperature (near room temperature) under vacuum.

The amount of H-resin vapor used in the process of this invention isthat which is sufficient to deposit the desired coating. This can varyover a wide range depending on factors such as the desired coatingthickness, the area to be coated, etc. In addition, the vapor may beused at nearly any concentration desired. If dilute vapor is to be used,it may be combined with nearly any compatible gas such as air argon orhelium.

The vaporized H-resin is then reacted to deposit the coating on thesubstrate. Reaction of gaseous species is well known in the art and anyconventional chemical vapor deposition (CVD) technique can be usedherein. For example, methods such as simple thermal vapor deposition,photothermal vapor deposition, plasma enhanced chemical vapor deposition(PECVD). electron cyclotron resonance (ECR). jet vapor deposition, orany similar technique may be used. These processes involve the additionof energy (in the form of heat, plasma, etc.) to the vaporized speciesto cause the desired reaction.

In thermal vapor deposition, the coating is deposited by passing astream of the vaporized H-resin over a heated substrate. When theH-resin vapor contacts the hot surface, it reacts and deposits thecoating. Substrate temperatures in the range of about 100°-1000° C. aresufficient to form these coatings in several minutes to several hours,depending on the thickness desired. It is to be noted that environmentswhich facilitate the desired deposition can also be used in thedeposition chamber. For instance, reactive environments such as air,oxygen, oxygen plasma, ammonia, amines, etc. or inert environments mayall be used herein.

In PECVD. the vaporized H-resin is reacted by passing it through aplasma field. The reactive species thereby formed are then focused atthe substrate and readily adhere. Generally, the advantage of thisprocess over thermal CVD is that lower substrate temperature can beused. For instance, substrate temperatures of about 20° up to about 600°C. are functional.

The plasmas used in such processes comprise energy derived from avariety of sources such as electric discharges, electromagnetic fieldsin the radio-frequency or microwave range, lasers or particle beams.Generally preferred in most plasma deposition processes is the use ofradio frequency (10 kHz-10² MHz) or microwave (0.1-10 GHz) energy atmoderate power densities (0.1-5 watts/cm²). The specific frequency,power and pressure, however, are generally tailored to the equipment.

Although the mechanism for this deposition is not clearly understood, itis postulated that the addition of energy to the vaporized H-resininduces reaction with oxygen and causes some rearrangement of themolecules so that a silicon and oxygen containing coating is formed onthe substrate. These coatings generally comprise primarily silica, butthey may also contain silicon suboxides, and residual Si--OC and/orSi--OH.

The process of this invention can be used to deposit desirable coatingsin a wide variety of thicknesses. For instance, coatings in the range offrom about a monolayer to greater than about 2-3 microns are possible.These coating may also be covered by other coatings such as further SiO₂coatings, SiO₂ /modifying ceramic oxide layers, silicon containingcoatings, silicon carbon containing coatings, silicon nitrogencontaining coatings, silicon nitrogen carbon containing coatings,silicon oxygen nitrogen containing coatings, and/or diamond like carboncoatings. Such coatings and their mechanism of deposition are known inthe art. Many are taught in U.S. Pat. No. 4,973.526, which isincorporated herein by reference.

The choice of substrates to be coated is limited only by the need forthermal and chemical stability at the temperature and in the environmentof the deposition vessel. Thus, the substrate can be, for example,glass, metal, plastic, ceramic or the like. It is particularly preferredherein, however, to coat electronic devices to provide a protective ordielectric coating.

The following non-limiting example is provided to illustrate theinvention to those skilled in the art.

EXAMPLE 1

Hydrogen silsesquioxane resin made by the method of Bank et al., U.S.Pat. No. 5.010.159, was fractionated using the process of Example 1 inU.S. patent application 07/618.865 and a fraction with a Mw Peak=437,Mn=361, Mw=414, Mz=459 and D=1.14 was collected.

The H-resin fraction was placed at one end of a tube furnace and asilicon wafer placed at the other end. 2 temperature zones were thenestablished in the furnace - a zone of about 100°-200° C. at the H-resinend and a zone of about 325°-415° C. at the wafer end. A flow of air wasestablished through the furnace from the end which held the H-resin tothe end which held the wafer. By this process, any H resin which wasvolatilized in the low temperature zone was carried to the hightemperature zone where it reacted and was deposited on the heatedsilicon wafer. The air flow was maintained for about 10 minutes. Afterthe air flow was stopped, the furnace was cooled and the wafer removed.

Examination of the wafer showed that a thin coating had been deposited.FTIR analysis of the coating showed the typical silica band at 1062cm⁻¹.

That which is claimed is:
 1. A method of depositing a coating containingsilicon and oxygen on a substrate comprising:introducing a gascomprising hydrogen silsesquioxane into a deposition chamber containingthe substrate; and inducing reaction of the hydrogen silsesquioxane bythe addition of energy to form the coating.
 2. The method of claim 1wherein the hydrogen silsesquioxane comprises a sample in which greaterthan about 75% of the species have a molecular weight less than about2000.
 3. The method of claim 2 wherein the hydrogen silsesquioxanecomprises a sample in which greater than about 75% of the species have amolecular weight less than about
 1200. 4. The method of claim 3 whereinthe hydrogen silsesquioxane comprises a sample in which greater thanabout 75% of the species have a molecular weight between about 400 andabout
 1000. 5. The method of claim 1 wherein the hydrogen silsesquioxaneis diluted in a carrier gas.
 6. The method of claim 1 wherein thehydrogen silsesquioxane is induced to react by exposure to an elevatedtemperature.
 7. The method of claim 6 in which the temperature is in therange of about 100°-1000° C.
 8. The method of claim 1 wherein thehydrogen silsesquioxane resin is induced to react by exposure to aplasma.
 9. The method of claim 8 in which the plasma comprises radiofrequency energy or microwave energy.
 10. The method of claim 1 in whichthe deposition chamber contains a reactive environment selected from thegroup consisting of air, oxygen, oxygen plasma, ammonia, and amines. 11.The method of claim 1 in which the substrate comprises an electronicdevice.
 12. The method of claim 1 additionally comprising applyingcoatings selected from the group consisting of SiO₂ coatings. SiO₂/modifying ceramic oxide layers, silicon containing coatings, siliconcarbon containing coatings, silicon nitrogen containing coatings,silicon nitrogen carbon containing coatings, silicon oxygen nitrogencontaining coatings, and diamond like carbon coatings on the coatedsubstrate.